This invention relates to a method for making a polyolefin shrink film having a reasonably balanced lower shrink tension in the machine direction (i.e. MD) and the transverse direction (i.e. TD). More specifically, this invention relates to a method for treating a biaxially oriented heat-shrinkable polyolefin film to partially relax the film, thereby significantly reducing the shrink tension in both the MD and TD, to produce a balanced lower shrink tension film, while maintaining a relatively high shrink percent.
2. Background Art
U.S. Pat. No. 3,891,737, granted June 24, 1975 to Marsh and Sargent, discloses a method for making a preferentially oriented heat shrinkable film of polyethylene or polyethylene containing copolymers, which method is based upon narrowly restricting the stretch ratio (the relative speeds of the stretch roll and the anneal roll) between 1.2:1 and 1.3:1. In addition, Marsh and Sargent use a series of four rolls, the first three hot and the fourth at room temperature.
U.S. Pat. No. 4,277,594, granted July 7, 1981 to Matthews et al, discloses a method for making a mildly biaxially oriented, blown polyethylene film by successively drawing and relaxing an ordinary biaxially oriented, blown polyethylene film in the longitudinal direction using a series of hot rolls turning at the appropriate speeds to achieve the desired degree of draw and relaxation, respectively. The Matthews process also requires passing the film through a transverse stretcher in between the longitudinal drawing and relaxing.
U.S. Pat. No. 3,141,912, granted July 21, 1964, to Goldman et al, discloses a process for preparing a biaxially oriented heat-shrinkable polyolefin film by extrusion of a tube of film, quenching the tubular film, maintaining a pressure within the tube, heating the film to the orientation temperature range, expanding the diameter of the tube at least 2X and advancing the tubular film in the longitudinal direction by at least 2X. This process can be used to prepare film which was actually used in each of the examples in this application.